1. Field of the Invention
The present invention generally relates to the field of handwriting interface devices for computing systems and, more particularly, to a switch for pen-down sensing in electronic styli.
2. Description of the Prior Art
The use of a stylus of some form for the creation of writing is as old as writing itself. The form of the stylus has been dictated by the writing medium. In modern times, the use of the highly refined skills involved in the use of writing instruments has been envisaged as a proper set of skills for use when communicating with a computing system. This is a natural outcome of the increased use of computing systems as a storage medium for documents, the even greater power of computing systems to perform actions that resemble human recognition, and the great flexibility that the stylus offers as an expressive tool. It should also be noted that for some written languages, such as those using Kanji characters, the stylus represents a far more acceptable tool for character entry than a keyboard.
The use of stylus input to computing systems has been used for such diverse applications as computer aided design (CAD), editing, spread sheet operation, signature recognition systems, and user interface operation. In each application, the stylus forms part of a "digitizer", the function of which is to convert stylus positions into computer-read numbers. Digitizers find their largest market in the area of CAD applications, so the requirements of the CAD applications have more or less determined the kind of switches used in digitizer styli.
In many CAD applications, the stylus is used in conjunction with a two-dimensional digitizing tablet to indicate a succession of static points the positions of which are recorded by the computer for further processing. Styli which have been built for CAD applications have switches incorporated in their structure to permit the user to indicate the to the computing system that the user is satisfied that the stylus is pointing to exactly the position that the user wishes to be recorded. The optimal switch for such use permits the user to make contact with the digitizer surface and even to apply substantial pressure to the stylus without actuating the switch. The very deliberateness which is a feature of these switches is a handicap in the area of handwriting capture.
When CAD oriented styli are used for handwriting data capture, it is not uncommon to find skips or other errors in the data stream. These are the consequence of the unaccustomed force and point travel required when such styli are used for writing. In the field of signature verification, considerable attention has been given to the development of styli which more closely simulate a writing pen so as to make the handwriting more natural, and hence the data input to the computer more accurate.
Recent developments in the field of automatic signature verification such as exemplified by U.S. Pat. No. 3,983,535 to Herbst et al. and U.S. Pat. No. 4,128,829 to Herbst et al. make the concept of personal identification via computer based signature analysis practical. The invention disclosed in U.S. Pat. No. 3,983,535 is based on the discovery that the accelerations of the stylus, which are proportional to the muscle forces exerted by the signer, are of predetermined consistent durations when forming particular strokes in a habitual signature. The nature of the process gives rise to various distortions in the time axis; e.g., pauses between sections of the name, skipped strokes, decorative rubrics, and the like. Thus, the signal is marked by regions of high correlation of unknown duration separated by variable regions of low correlation. Accordingly, the invention in U.S. Pat. No. 3,983,535 dealt with a method of regional correlation which registered these regions based initially on stylus contact and then shifting the regions individually to find the maximal of the correlation function weighted to penalize shifting. The results were then combined to make an overall verification decision.
The signature verification method disclosed in U.S. Pat. No. 3,983,535 was based on a single acceleration parameter of a signature dynamic, but as disclosed in U.S. Pat. No. 4,128,829, an even greater discrimination in the verification operation is possible using two orthogonally disposed (e.g., x and y axes) acceleration components together with the pressure patterns which are produced during the writing of the signature and utilizing all three of these individual parameters in the correlation operation. The invention discloses in U.S. Pat. No. 4,128,829 retained the concept of segmenting the sample and utilizing a series of successive shifts to obtain the maximum possible correlation, weighting the correlations, and finally combining the individual correlation statistics for all segments. An example of a pen that may be used in the Herbst et al. verification system is disclosed in U.S. Pat. No. 4,142,175 to Herbst et al. This pen produces electrical signals proportional to accelerations in the x and y axes and an electrical signal proportional to the pen point pressure along the z axis.
It is to understood that the stylus described above contains apparatus which is specialized to the operation of a signature verification system. The accelerometers provide a data stream from which relative changes (not absolute changes as with a digitizer) in position may be derived. Since the pressure gauge must be able to assume several states to allow detection of several pressure levels, its travel can not be arbitrarily small. Hence, it is a less than perfect emulation of a writing instrument.
The data stream produced by an electronic stylus used for the purpose of handwriting input to a computer should not be distorted by user hand fatigue or other effects introduced by the stylus ergonomics. While writing instruments, such as pens and pencils, have mechanical properties to which their users become accustomed, these properties are not easily replicate in an electronic stylus. The act of writing involves a complex collection of both guided and reflexive actions which can be disrupted by substantive change in the properties of the stylus. The weight, thickness, drag, shape, and other gross mechanical properties of a writing instrument can also influence the fatigue generated by the act of writing. Fatigue can in turn introduce its own form of distortion into the user's writing.
In an environment in which a computing system is used to interpret handwritten characters and gestures, regularity in the produced handwriting is needed to keep the recognition algorithm simple and to keep recognition error to a minimum. Fatigue and the need for increased mental effort on the part of an electronic stylus user mitigates against this regularity and should be minimized. To the extent that it is possible, this objective can be achieved by the designing of an electronic stylus to imitate the mechanical properties of the conventional mechanical stylus.
Three properties of pens' and pencils' "activation" must be imitated by an electronic stylus to provide its user with a similar experience. A pen or pencil need not be pressed forcefully into paper to cause marking to begin. The point of such a stylus is not displaced axially to cause writing to begin. The point of such a stylus is resistant to being displaced axially. Conventional electro-mechanical switches provide a very poor fit to the problem of sensing electronic stylus contact while imitating conventional stylus properties. Switches which are used in common electronic styli can be characterized as having significant "pre-travel", "differential-travel", and "post-travel", as well as having activation force requirements. Pre-travel is the distance that a switch actuator must be pushed before there is any chance it will "close". Differential-travel is the distance over which switch closure is uncertain. Post-travel is the distance a switch actuator travels after switch closure has been achieved. Pre-travel, differential-travel, and post-travel are inconsistent with the zero-axial-travel requirements for imitating conventional writing instruments such as pens and pencils. Activation pressures higher than used in writing are inconsistent with the low-force requirement.
Many of the alternatives to electro-mechanical switches share their drawbacks or introduce new ones. Switches employing strain gauges, for example, must use substantial electronic systems to produce a switch simulation with no hysteresis. These electronic systems may require individual adjustment to compensate for differences between individual strain gauges.
U.S. Pat. No. 4,695,680 to Kable discloses a pen-down sense switch which utilizes a Hall effect sensor and a pair of permanent magnets to implement a switch. The magnets are arranged within the stylus so that they produce a magnetic field with a large gradient, and so they are moved by the retraction of the stylus point caused by the stylus body being pushed toward the tablet. Movement of the magnets causes the magnetic field intensity to change in the Hall sensor, which switches state when the magnetic field intensity change is sufficient to indicate the pen condition (up or down).
U.S. Pat. No. 4,034,155 to Muller et al. discloses a pen-down sense switch which utilizes the retraction of the stylus point to move a barrier into the optical path between a light source and a light detector. The barrier must travel in order to arrive at the critical point in the light path so switching does not take place at the start of travel and cannot be arbitrarily small. In general, electro-optical-mechanical switches have fundamentally the same properties as electro-mechanical switches.
Also known in the prior art are U.S. Pat. No. 4,848,496 to Murakami et al. and U.S. Pat. No. 4,786,765 to Yamanami et al. In U.S. Pat. No. 4,848,496, no switch technology per se is taught; rather, this patent discloses a method for determining the open a or closed state of a switch through its measurable effects on a tuned circuit. In U.S. Pat. No. 4,786,765, pen down detection is performed either by means of external sensing of the effects of switch state on the externally measurable behavior of a tuned circuit or by mechanically coupling the stylus point to either a resistance, capacitance or inductance element of the externally measured tuned circuit. This mechanical coupling will affect a change in tuning of the resonant circuit which, on passing some threshold value, will be encoded by the detecting circuitry as a pen up or down state.
In all of these prior art stylus designs, the stylus point is required to travel some distance before a change of state can be reliably detected; i.e., there is no reliably detectable change of state at the start of travel. Some digitizing technologies provide an intrinsic pen down sense as in the case of conductive sheet digitizers, but these may not be the digitizers of choice for a given design. Further, such digitization techniques may be inaccurate at very low stylus pressures and so may be usefully supplemented by a suitable stylus switch.